Shelling is a term sometimes employed by those familiar with the abrasive industry, particularly the coated abrasive industry, which refers to the phenomenon whereby abrasive grains are released prematurely from a bond system which is typically intended to hold the abrasive grain throughout the useful life of the abrasive grains. The term "shelling" may have been adopted because someone thought the phenomenon of the abrasive grains being released from the bond system was similar to the releasing of kernels of corn from an ear of corn during a corn shelling operation. While there are some exceptions, e.g., where the abrasive product is used as a source of abrasive grain for a slurry, shelling of abrasive grain from abrasive products is unwanted because it has the effect of reducing the efficiency of the abrasive product because of the loss of the abrading surfaces which would have been provided by the missing abrasive grain, and for other reasons explained below. The decreased efficiency by shelling can be noted in all types of abrasive products such as bonded abrasive products, e.g., grinding wheels, and non-woven abrasive products, but is particularly notable in coated abrasive products where substantially all of the abrasive grains are held on a sheet by the bond system with one end of each grain typically being exposed or nearly exposed. The loss of abrasive grain by shelling from coated abrasive products can provide non-abrasive areas on the coated abrasive surface, reducing abrasive efficiency and possibly resulting in uneven surface finishing.
The shelling problem can be particularly significant when the abrasive product is used for high stock removal applications. During such use, the abrasive product must be able to withstand high pressures and rotative speeds and still provide sufficient abrasive cut. For example, a coated abrasive disc can traverse 12,000 revolutions per minute (rpm) and be subjected to an interface pressure as high as 15 kg/cm.sup.2. While these severe conditions are preferred because they usually result in increased cut rates, they also severely tax the adhesive bond between the abrasive grain and the bond system. If this bond fails, abrasive grains are ejected (or shelled) from the coated abrasive product at an extremely high rate of speed which could result in serious injury to the operator, particularly when the abrasive grains are of a large grit size. This hazard is of grave concern to those in the abrasives industry and it sometimes results in products not being used to their full potential because of the possible safety hazard.
Furthermore, many abrasive products, particularly coated abrasive products, utilize moisture-susceptible resinous bond systems, the most popular comprising phenolic resin. Bond systems based upon phenolic resins are known to increase the potential for an abrasive product to shell as the moisture level of the grinding conditions is increased.
Many solutions to the shelling problem have been proposed, but they have been either impractical, expensive, of little utility, or not particularly suited for use with sintered ceramic abrasive grains.
The earliest known reference specifically directed to solving the shelling problem is U.S. Pat. No. 1,528,543 (Hartmann) which discloses a process for treating the surfaces of crystalline mineral materials to scratch and roughen the grain surfaces and thus increase their bonding qualities. Various other references disclose the adhesion of smaller particles to the surface of larger particles to increase surface area by utilization of a bonding layer or flux to obtain particles having surface bonded smaller particulate material. For example, U.S. Pat. No. 3,269,815 (Koopman) discloses coating abrasive grain with finely divided particles of solid material such as carbides and nitrides by cementing these particles to the abrasive grain by a thin ceramic film such as glass. The thin ceramic film will soften at a temperature less than the softening or melting of the abrasive grain or the solid materials to thereby cement the finely divided particles to the abrasive grain to promote resin adherence and increase the bonded strength between the particles of abrasive grain when in a bonded abrasive article. Additionally, U.S. Pat. No. 4,472,173 (Bruning et al) discloses corundum abrasive grain which is coated with ground frit, a binder and fine grain highly abrasive material for the purpose of improving adhesion of the abrasive grain in its processing to resin-bonded abrasives.
Additionally, certain references disclose forming compacts of smaller particles which would inherently have a greater surface roughness. For example, U.S. Pat. No. 4,252,544 (Takahashi) discloses alumina abrasive grain constructed of electro-fused or high temperature calcined alumina coarse crystal particles and alumina fine crystal particles which are located between the alumina coarse crystal particles. These abrasive grains are manufactured by forming alumina coarse powder of a particular type, forming alumina fine powder of a particular type, kneading the alumina coarse powder and alumina fine powder in the presence of water or, if necessary, primary binder, extruding the kneaded material by means of a mechanical extruder, drying the extruded material, cutting the extruded material to a predetermined length, and sintering the dried and cut pieces of the extruded material at a temperature higher than that of the calcining temperature of the fine powdered alumina and lower than 1700.degree. C.
There is no indication in any of the aforementioned references that the teachings could be utilized to produce alpha-alumina based ceramic abrasive grain having desirable physical properties without altering the same and without modifying the ceramic abrasive grains by the application of binder layers or flux. The type of ceramic abrasive grain desired to be improved according to the present invention is, for example, that disclosed in assignee's U.S. Pat. No. 4,744,802 (Schwabel). Other references which disclose the preparation of alumina-based ceramic abrasive grain of this type include U.S. Pat. Nos. 4,314,827 (Leitheiser et al), 4,518,397 (Leitheiser et al) and 4,574,003 (Gerk).
None of the above mentioned references discloses ceramic particulate material having small separated masses of inorganic material autogenously bonded to the surface thereof to provide an improved abrasive grain which is shelling resistant, or abrasive products such as bonded abrasive products, non-woven abrasive products, and coated abrasive products, which contain the same.